1. Field of the Invention
This invention relates to ion micro-analysis especially capable of measuring and indicating the analyzed depth during the analysis.
2. Description of the Prior Art
In ion micro-analysis, the surface of a specimen is bombarded with a primary ion beam and secondary ions generated thereby are mass-analyzed. Accordingly, through ion micro-analysis, composition of the specimen, content of specific atomic (molecular) species and the like at the location on which the primary ion beam is bombarded can be determined. When the specimen surface is scanned with the primary ion beam, the distribution of specific secondary ions in the specimen surface can be determined. Further when the same area of a solid specimen is successively analyzed, the specimen is sputtered (etched) and the analysis gradually proceeds to deeper locations so that distribution of specific secondary ions in the direction of depth can be determined. Therefore, ion micro-analysis has versatility especially in analysis of small quantities and analysis in the direction of depth, and it is highlighted in the field of semiconductor technology and metallography. For general knowledge of the ion micro-analysis, reference is made to "Analytical Chemistry" by Charls, A., Evans, Vol. 44, No. 13 (Nov. 1972) pp 67A-80A.
U.S. Pat. No. 3,881,108 entitled "Ion Microprobe Analyzer", issued in Apr. 29, 1975 to Toshio Kondo and Hifumi Tamura and assigned to the present assignee proposes an electronic aperture technique. According to this proposal, in order to assure uniform etching and eliminate influence of noise components stemming from the periphery of a crater formed by etching, a primary ion beam is scanned stepwise within a predetermined specimen area, and only secondary ions generated from a central portion of the scanned area are detected. In other words, the sampling region is confined within part of the etched region to improve reliability of signal detection.
U.S. Pat. No. 3,494,233 entitled "Ion Microprobe Analyzer", issued on July 8, 1975 to Hifumi Tamura, Toshio Kondo and Kazumitsu Nakamura and assigned to the present assignee proposes a total ion monitoring technique. According to this proposal, a monitoring signal corresponding to intensity of the total secondary ions is detected and the detected intensity of specific secondary ions is divided by the monitoring signal, thereby compensating for fluctuations in the secondary ion intensity due to variations in primary ion current, etching rate, secondary ion collecting rate and the like factor. In other words, fluctuations in signal levels due to variations in analyzing conditions are compensated for improving accuracies of the quantitative analysis.
For presenting results of the ion microanalysis in the direction of depth (depth analysis), secndary ion intensity distribution or specimen composition is preferably presented as a function of the depth. When the etching rate is constant, the depth of analyzed site or location can be determined from etching time and the depth of a crater (etching depth) after the analysis. For measurement of the crater depth after the analysis, a multiple interferrometer, Taylor-Hobson type surface roughness meter or the like instrument may be used. By using a mean sputtering rate obtained by dividing the measured etching depth by time of primary ion beam bombardment, a depth corresponding to each analyzed site can be obtained. In these methods, the measurement of depth must be carried out after completion of the microanalysis, and it is difficult to determine etching depths (analyzed depths) during progress of the analysis. A way of estimating the etching depth during the analysis is available wherein sputtering yield (atoms/ion) is measured in advance and the sputtering rate is calculated from the measured sputtering yield and primary ion bombardment conditions.
It is known that the sputtering yield by the primary ion bombardment depends upon primary ion species, primary ion energy, incident angle, current density and degree of vacuum in an analyzing chamber and that it changes with the element of a specimen and impurity concentration in the specimen. Of these factors, the current density changes in a complicated manner with gas pressure and temperature of an ion source and ion beam size and it is difficult to always place the current density in the same condition. Accordingly, the this method it is difficult to improve the accuracy of estimation of the etching depth. Failure to determine the etching depth during the analysis leads to the need for unnecessarily excessive tolerance and forces extra etching (analysis) of the specimen, resulting in a waste of analysis time and additional cost. Therefore, it is desired to determine the analyzing depth with high accuracies during the analysis.